This investigation studied whether on demand fluid intake during exercise induced sweat loss at varying rates during the menstrual cycle. Females (N = 7) underwent three exercise-rest trials (30 minutes of treadmill walking, 3% grade, 30-35% VO2max, 30 minutes of sitting during a three-hour period in 36°C) during three different phases of the menstrual cycle (menses, midfollicular, midluteal).

Total fluid intake was similar in all phases. Stomach fullness ratings were higher during the luteal phase than during the other two. Basal plasma osmalality was lower in the luteal phase than in the midfollicular phase. There were no effects on perceived thirst ratings, plasma vasopressin concentrations, or plasma volume across the duration of the study.

Implication. Voluntary fluid intake is not affected by the menstrual cycle phase when exercising in the heat.